The U.S. Army Corps of Engineers and others involved in hydropower generation are actively developing techniques to move juvenile salmon past generators without entraining the fish through turbines (see, e.g., Northwest Power Planning Council, Columbia River Basin Fish and Wildlife Program, Portland, OR, 1994; National Marine Fisheries Service, "Biological opinion: reinitiation of consultation on 1994-1998 operation of the federal Columbia River power system and juvenile transportation program in 1995 and future years." 1995). Constructing fish bypasses around hydroelectric projects can cost millions of dollars, and yet the bypasses may not work as well as their designers had hoped. To develop methods for improving the survival of fish near hydroelectric projects, it is important to evaluate the behavior of the fish as they approach and pass through a hydroelectric project. In studying the behavior of fish near hydroelectric projects, it is desirable to be able to simultaneously track the position of multiple fish throughout an area in front of a fish bypass.
Conventional methods of tracking underwater objects are useful in many instances but suffer from various limitations. For example, conventional rotating tracking sonars collect data over relatively small areas. Thus, they may not provide a picture of fish behavior over the entire area of interest. Additionally, conventional methods are prone to biases such as the propensity of the systems to lock onto large fish, rather than small ones, and to track the large fish for a long period of time. This characteristic prevents the acquisition of new targets during the tracking event, and results in relatively small sample sizes over a given time period.
As compared to conventional sonars, multibeam sonars offer a relatively unbiased, coherent view of the water column. Other researchers have explored the use of a single head multibeam sonar to study the behavior of bottlenose dolphins (Ridoux et al., "A Video Sonar As A New Tool To Study Marine Mammals In The Wild: Measurements of Dolphin Swimming Speed," Marine Mammal Science, 13(2): 196-206 (1997)). This study included estimation of swimming speed relative to the bottom and to the tidal current. The study did not include the use of plural sonars or the tracking of fish in three-dimensions (3-D).
A non-conventional method for sonar detection was disclosed by Edgerton et al. in U.S. Pat. No. 4,121,190. In this method, underwater objects are detected using two single beam sonar arrays that are positioned orthogonally and that transmit sound waves from each sonar array. Objects that move through the relatively narrow areas sampled by either beam are detected and recorded in analog form on a chart recorder. The system described by Edgerton et al. could only pinpoint an object in three dimensions when the object was located at the point where the horizontal and vertical beams intersect. The system could not be used to track objects in 3-D. It was also disadvantageous because it used at least two transmitters to ping, and thus the combined beam pattern would be distorted due to interference.
Thus, there is a need for an apparatus and method for unbiased 3-D tracking of fish.